Merge Multiple CSV Files Online

To merge CSV files online, upload two or more CSV files to Large CSV Viewer and choose Merge from the File menu. You can combine files vertically (stacking rows) or join them horizontally by a shared key column. Column headers are aligned automatically. The merged file downloads as a single CSV. Free, no account, no size limits.

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Combine multiple CSV files into one online for free. Upload 2 or more CSV files and merge them vertically or by a shared key column. No install needed.

Working with data split across multiple CSV exports? Large CSV Viewer lets you upload multiple CSV files and combine them — either stacking them vertically into one dataset (UNION) or joining them side by side on a shared key column (JOIN). No coding, no spreadsheet software required.

Key features

  • Merge 2 or more CSV files into a single dataset
  • Vertical merge (UNION): stack rows from all files
  • Horizontal join (JOIN): combine columns by a shared key
  • Column headers are aligned automatically across files

How it works

  1. Upload your CSV files. Upload the first CSV file, then add additional files using the 'Add file' button. Up to 10 files can be merged at once.
  2. Choose merge type. Select Vertical (stack rows) or Horizontal (join on a key column). For horizontal joins, select the shared key column from the dropdown.
  3. Download the merged file. Click Merge and your combined CSV file downloads immediately. All headers from both files are preserved.

Ready to try it?

Free, instant — no account or install needed.

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Frequently asked questions

What is a CSV merge?

A CSV merge combines two or more CSV files into a single file. A vertical merge stacks rows from all files; a horizontal merge joins files side by side based on a shared column like an ID.

What happens when the CSV files have different column headers?

For vertical merges, all unique column headers are included in the output. Rows from a file that don't have a particular column will have empty values for that column.

Can I merge large CSV files?

Yes. Merging runs server-side using DuckDB so it works on files of any size without crashing your browser.

How many files can I merge at once?

Up to 10 CSV files can be merged in a single operation.

Is the CSV merge tool free?

Yes, completely free. No account, no file size limit, no paywall.